1. Field of the Invention
The present invention relates to a cylinder block integrated with a crankcase, which is a single cast block integrating a cylinder part and a crankcase part, particularly a crankcase-integrated cylinder block of a small internal combustion engine reinforced in strength and rigidity for reducing vibration and noise.
2. Description of the Related Art
As is well known, the main body of a small single cylinder internal combustion engine consists of a cylinder part in which a piston reciprocated and a crankcase part in which a crankshaft is supported for rotation. A cylinder head is attached to the top end of the cylinder part and a crankcase cover is attached to the crankcase part at an opening in one side of thereof to cover the opening. In small internal combustion engines, the cylinder part and crankcase part are cast in one unit block. Such a unit block is called a crankcase-integrated cylinder block.
In such a crankcase-integrated cylinder block (hereafter referred to as a cylinder block), endeavors have been continued to reduce thickness of walls of the cylinder block from the point of view of downsizing and weight saving of the internal combustion engine. However, there occur often problems when thickness of walls of the cylinder block is reduced that the strength/rigidity thereof reduces, vibration is increased, resulting in occurrence of cracks and increase in noise.
When designing a cylinder block, a fundamental plan is established through studying strength and vibration characteristic by calculation, however, the structure of the cylinder block is very complicated three-dimensionally and difficult to analyze. Designing labor has been reduced by calculating using a computer by applying a finite-element method, however, the calculation enables evaluation of a cylinder block of specific shape only, and it is necessary to compare calculation results of a variety of constructions of cylinder blocks in order to carry out optimization of cylinder block configuration. Therefore, designing of a cylinder block remains yet in a state that a vast amount of labors is needed for calculation, and here remain problems concerning designing cost and design schedule.
To solve the problems, there is disclosed in Japanese Patent No.3087106 an art to provide a crankcase-integrated cylinder block with reduced wall thickness resulting in decreased weight of engine, without entailing reduction in strength and increase in noise, and enabling reduction of design cost for optimizing the configuration of cylinder block.
According to the prior art like this, the cylinder block is constructed such that large force bearing portions such as a base part (fixing foot part) for installing the engine and bearing parts for supporting the crankshaft are thick, and forces exerting to such thick portions are transmitted from the thick portions to thin walls in a state distributed to ribs, etc. To be more specific, as shown in FIG. 5(B) in the patent specification, a first bearing boss having a first bearing hole for inserting a first bearing is provided on the central part of the crankcase cover to protrude inwardly, a second bearing boss having a second bearing hole for inserting a second bearing is provided on the central part of the backside wall of the cylinder block to protrude inwardly, a pair of upper side boss-ribs is provided to extend from the upper part of each of the bearing bosses widening the distance between each of the boss-ribs upwardly, and a pair of lower side boss-ribs is provided to extend from the lower part of each of the bearing bosses widening the distance between each of the boss-ribs downwardly.
However, in the art, the trapezoidal region surrounded by the upper side boss-ribs of the first bearing boss, upper rib of the crank case cover, and a part of the outer periphery of the first bearing boss, and the trapezoidal region surrounded by the upper side boss-ribs of the second bearing boss, upper rib of the cylinder block, and a part of the outer periphery of the second bearing boss, must be formed to be thick. Therefore, portions where large force is supported such as a base part (fixing foot part) to install the engine and bearing parts for supporting the crankshaft are formed thick, and thin-walled cylinder block configuration is not attained fully enough.
Further, application of the prior art is restricted to a crankcase-integrated having a vertical cylinder and can not be applied to a crankcase-integrated cylinder block having an inclined cylinder.
It is difficult to apply said prior art to an inclined cylinder type engine such as disclosed for example in Japanese Patent No.3243555 in which a crankcase-integrated cylinder block which has a cylinder part extending from an inclined wall part of the crankcase part perpendicular thereto and of which the cylinder axis is inclined from the direction perpendicular to the base part for installing the engine.